In the science of photoplethysmography, light is used to illuminate or trans-illuminate living tissue for the purpose of measuring blood analytes or other hemodynamic or tissue properties. In this monitoring modality light is injected into living tissue and a portion of the light which is not absorbed by the tissues, or scattered in some other direction, is detected a short distance from the entry point. The detected light is converted into an electronic signal that is indicative of the received light signal from the tissue. This electronic signal is then used to calculate physiologic parameters such as arterial blood oxygen saturation and hemodynamic variables such as heart rate, cardiac output, or tissue perfusion. Among the blood analytes that may be measured by photoplethysmography are the various species of hemoglobin, including the percentages of oxyhemoglobin, carboxyhemoglobin, methemoglobin, and reduced hemoglobin in the arterial blood. A device which detects and processes photoplethysmographic signals to measure the levels of various blood analytes and hemodynamic parameters is referred to as a photoplethysmographic apparatus, device, or instrument. Typically these instruments also include, and control, the light sources or emitters used to generate the light that illuminates the tissue. The signals received from the tissue are referred to as photoplethysmographic signals.
The first widespread commercial use of photoplethysmography in medicine was in the pulse oximeter, a device designed to measure arterial blood oxygen saturation. To make these measurements, two different bands of light must be used, with each light band possessing a unique spectral content. Each spectral band, or light band, is typically referred to by the center wavelength, or sometimes by the peak wavelength, of the given band. In pulse oximetry two different light emitting diodes (LEDs) are typically used to generate the sensing light, one with a center, or peak, wavelength near 660 nanometers (nm) and a second with a center, or peak, wavelength near 940 nm.
Light from each LED light source, or emitter, is passed into the tissue-under-test, usually a finger, earlobe, or other relatively thin, well-perfused tissue sample. After passing some distance through the tissue-under-test, a portion of the light not absorbed by the tissue or scattered in some other direction is collected by a photodetector and converted into electronic signals that are directly proportional to the received light signals. The channels, or electronic signals from each of the different light sources, are kept separated through the use of any one of a number of different well-published techniques, including but not limited to, time-division multiplexing or frequency-division multiplexing.
Typically, photoplethysmography is conducted using one pulse oximeter probe. The raw signal stream obtained from a pulse oximeter probe is related to the amount of light from the LED that hits the photodetector of the pulse oximeter probe. The magnitude of the signal from the photodetector is inversely proportional to the amount of absorption of the light between the LED and the photodetector (greater absorption results in less light exciting the photodetector). The absorbed light is due to multiple factors, including absorption due to tissue, absorption due to venous blood, absorption due to arterial blood, and absorption due to the pulsation of arterial blood with each heart beat. Typically, the raw signal from the photodetector is processed (e.g. removal of artifacts and autogain of the signal) in order to obtain an arterial oxygen saturation value and the plethysmograph is largely ignored. While some attempts have been made to process photoplethysmography signals, these attempts have focused on the ‘cleaning’ of signal for purposes of removing motion artifacts. The issue of identifying unique and helpful information that might be included in the photoplethysmography stream has been left largely unexplored.